Absorption spectroscopy methods are methods for assaying components of a material responsive to absorption of electromagnetic radiation by the material. Common to practice of many of the methods, a material to be assayed is exposed to electromagnetic radiation at a plurality of different wavelengths of the radiation. At each wavelength, absorption of the radiation is directly or indirectly measured to determine its absorption by the material as a function of wavelength. The absorption of the electromagnetic radiation as a function of wavelength is referred to as an “absorption spectrum” for the material.
The absorption spectrum comprises contributions from absorption spectra of atoms and molecules that the material contains, each of which has its own unique absorption spectrum. The absorption spectrum for the material is processed to identify absorption spectra of atoms or molecules that contribute to the material's absorption spectrum, and amounts by which they contribute to the absorption spectrum. The identified spectra and amounts are used to identify and assay atoms and molecules that the material comprises.
Absorption spectroscopy is typically used to assay materials in a gaseous state. Absorption spectroscopy of solids in their naturally occurring states under ambient conditions of atmosphere and temperature generally suffers from spectral line broadening that makes it complicated and difficult to use absorption spectroscopy to assay components of the solids or liquids. A spectral line of an atom or molecule marks a generally narrow band of wavelengths (or frequencies) in the electromagnetic spectrum associated with a difference between two different energy states of the atom or molecule at which it absorbs or emits energy. Every atom or molecule has its own unique family of spectral lines. The family of spectral lines, defines, the atom's or molecule's emission and absorption spectrum. When referring to absorption of electromagnetic energy by the atom or molecule, the family of spectral lines is referred to as the atom's or molecule's absorption spectrum. Broadening of a spectral line of an atom or molecule refers to an increase, a “broadening”, of the range of energies, and therefore of wavelength band, at which the atom or molecule can absorb energy to make a transition been energy states associated with the spectral line.
Spectral line broadening of an atom or molecule in a solid is generated not only in accordance with the Heisenberg uncertainty principle, which puts a lower limit on a spectral line width associated with a given state transition of an atom or molecule. Interaction of the atom or molecule with other components of the densely packed material characteristic of the solid, and Doppler shifts due to random thermal motion of the atom or molecule contribute to its spectral line broadening.
Spectral line broadening in a solid or liquid in a natural state and under ambient conditions is typically so large that spectral lines of different atoms and/or molecules in the solid tend to overlap substantially. It is therefore difficult, if at all practically possible, to identify an atom or molecule in a solid or liquid from an absorption spectrum acquired for the solid or liquid in its natural state and under ambient atmospheric and temperature conditions. As a result, absorption spectroscopy assaying of solids and liquid performed under ambient conditions has not generally been useful.